The detection of relatively heavy particle radiation is often rendered more difficult by the presence of gamma rays within the radiation field being measured. It is common for nuclear reactors to emit both gamma radiation and energetic neutrons in varying proportions each having different radiation exposure effects. Although the presence of gamma rays is typically of interest in this situation and in other forms of personnel dosimetry, the component of total radiation exposure attributable to heavy particle radiation is often of independent interest. The desire to independently measure heavy particle radiation stems from the differing effects associated with it as compared to other radiation forms with which it can occur. Heavy particle radiation is also of independent concern because of variations in the sensitivity of measuring techniques to these differing forms of radiation, or because of other non-linearities which occur in dose measuring techniques which by intention or nature simultaneously detect combined gamma radiation doses and heavy particle radiation doses.
Similarly, the effective dose for purposes of personnel dosimetry for some forms of heavy particle radiation, such as from neutrons, is non-linear. Thus there is a need for a dosimeter and dosimeter reading system which selectively detects or measures the radiation so that the radiation dose information can be placed in a form indicative of the effective dose received by a person working or otherwise exposed to these forms of radiation. When one or more types of radiation are measured using a single dosimeter detector, then such non-linear dose versus measurement relationships complicate the measurement and interpretation of the dosimeter and can lead to indeterminate radiation doses not attributable to the type of radiation involved. Accordingly, in order to provide more accurate personnel radiation dose information it is necessary in many radiation environments to independently measure both of these radiation components.
Heavy particle radiation is for purposes of this document radiation associated with relatively heavy nuclear particles emitted during radioactive decay and other radiation producing processes and phenomena. Such heavy particles have a molecular weight of approximately 1 dalton or greater, and include particles such as neutrons, protons, heavier forms of mesons, alpha particles (helium nuclei), deuterons (nuclei of heavy hydrogen, deuterium) and other still larger nuclear particles emitted at energy levels generally in excess of approximately 10 keV or greater. Heavy particle radiation should be distinguished from light particle radiation or non-particulate rays such as electrons, neutrinos, and gamma rays, and others which have masses of zero, near zero, or at most two orders of magnitude less than 1 dalton.
Heavy particle radiation can occur as heavy charged particles, such as protons, alpha particles, deuterons, some forms of mesons, and other relatively heavy nuclei or nuclear components which have been emitted without sufficient associated electrons to maintain a neutral electrical charge. Heavy particle radiation can also occur in the form of uncharged heavy particles such as neutrons, and some types of heavy mesons. Heavy particle radiation is typically of greater interest from a personnel dosimetry monitoring standpoint when the particles are ionizing radiation. In general, neutral particles such as neutrons do not themselves directly cause ionization because they are neutral bodies and do not cause electrons to be displaced as they strike tissue or other materials of interest. However, personnel dosimetry for neutrons is still important because energetic neutrons interact with hydrogen containing molecules to emit energetic protons which are ionizing radiation, the effects of which are dependent upon the energy levels of the stimulating neutrons and the protons being radiated thereby.
Prior systems for monitoring neutron radiation exposure have typically had difficulty measuring over a broad dynamic range of radiation levels in a manner which tracks the dose equivalency of the radiation. FIG. 1 shows a dose equivalency curve for personnel (human) neutron radiation dosimetry. This figure indicates that below 10 keV the energy level of the neutrons is of little significance in determining the effective dose for purposes of measuring radiation exposure. At neutron energy levels above 10 keV the increasing energy level of the neutrons has an increasing effect on human tissue and should be reflected in the radiation dose measurement to provide accurate assessment of radiation exposure. This increasing effect of neutron energy level increases up to energy levels of approximately 1 MeV where the curve essentially levels off and the effects of increasing neutron energy are not significant for personnel radiation dose monitoring purposes.
A number of approaches have been used to try and compensate for the non-linear relationship between effective neutron dose and the variations in measuring neutron radiation using different detection techniques. In one dosimeter design by Harvey a lithium fluoride dosimeter using a pair of dosimeters of phosphor types TLD-600 and TLD-700 were mounted in a 4.8 centimeter polyethylene sphere in an effort to match the dose responsive curve indicated in FIG. 1, for neutrons up to energy levels of 10 keV. Harvey indicated that an appropriately designed neutron to knock-on proton converter might meet the dose equivalency requirements above 10 keV, if a detector could be found which provides sufficient proton detection sensitivity down to energy levels of 10 keV. The only current technology which approaches this level is the C39 polycarbonate track etch dosimeter which does not demonstrate proton detection sensitivity sufficient to meet this minimum threshold. Current techniques for electro-chemical etching of the C39 polycarbonate dosimeters provide lower sensitivity thresholds in the 100-300 keV range.
A number of dosimetry systems are in use which combine albedo and track etch methods. The most sophisticated of these appears to be the Karlsruhe system which employs a 3 millimeter thick boron-loaded encapsulation for a pair of TLD-600 and TLD-700 lithium fluoride chips to provide albedo, reflected, radiation detection. These dosimeter elements are combined with Macrofol (polycarbonate) track etch dosimeters. The boron shield for this dosimeter has small collimating holes on the side which faces the body of the wearer to reduce the dependence of the dosimeter to body distance. Although these dosimeters represent the best available technology they still do not provide the desired level of accuracy in measuring heavy particle radiation as distinct from gamma radiation. They further do not provide the desired level of sensitivity to minimum energy levels for heavy particle radiation.
Of still further significance is the fact that these and other track etch dosimeters do not provide any variation in the measured dose dependent on the energy level of the heavy particle. Instead the track etch dosimeters all suffer from the very significant limitation that a heavy particle is either sufficiently energetic to cause a track to be identified or not. Once this sensitivity threshold is surpassed there is no practical ability to discern the energy level of the particle. This severely limits the potential accuracy of the track etch techniques and indicates the continuing need for dosimetry systems for heavy particles which can provide at least some indication of the relative heavy particle energy level of the radiation received during the exposure period.
It is also noteworthy that track etch detectors in general are insensitive to gamma rays. Although this property of these detectors is beneficial in selectively detecting heavy ionizing particles, it requires that a distinct dosimetric detector be included in any dosimeter or dosimeter badge used in personnel dosimetry where gamma rays also need to be measured. Since there is typically at least some risk from gamma radiation in almost all situations where personnel dosimetry is used, this necessitates more complex dosimeter configurations having at least two detector types to monitor both types of radiation. This in turn requires the manufacture of both dosimeter elements and assembly of both in a dosimeter element holder or badge. The reading of the dosimeter further requires that both dosimeters be read. If the technologies of the two or more dosimeter elements is different, then different laboratory or other dosimeter reading facilities must be used for each of the different dosimeter technologies used in each badge. This necessarily increases the cost of processing and reading the dosimeters. Accordingly, there are distinct advantages associated with dosimeter systems which can derive relatively independent measurements of two types of radiation, such as heavy particle radiation and gamma radiation, using a single dosimeter and dosimeter reading apparatus. It is even more preferable if such reading can be accomplished in a single reading operation.
The technologies employing chemical track etch dosimeters are further burdened by the required lengthy and costly treatment of the detectors in concentrated caustic solution to etch the radiation tracks into the polycarbonate or other track etch media used in the dosimeter detector. This disadvantageous and dangerous etch processing is exacerbated in electrochemical etching by the electrical shock hazard associated with using applied etching potential of up to 2000 volts. These hazards require special laboratory facilities capable of safely handling these processes which make many desired on-site dosimeter reading applications impractical. Because of these factors, track etch dosimeters are not acceptable for a number of important applications in personnel radiation dosimetry.
In addition to personnel monitoring there are other situations in which the level of heavy particle radiation is preferably measured without the potential or actual effects associated with gamma rays or other forms of non-heavy particle radiation. For example, in the detection of radon gas there is typically simultaneous exposure of the radiation detector to both gamma rays and the alpha rays which most appropriately indicate the decay of radon. Thus the person seeking to measure radon levels using a detector which is not selective to alpha particles versus gamma rays is confronted with the potentially erroneous effects of the gamma rays.
Current techniques for measuring radon gas include charcoal packets and canisters. These detectors saturate after only a few days of air sampling. They also rely on indirect gamma radiation rather than direct detection of alpha particles which are more accurate indicators of radon concentration. They further require up to one hour of laboratory measurement time with a scintillation counter to determine the radon concentration of the air sampled by the detector. The relatively smaller "tea-bag" type carbon packets absorb radon so rapidly that equilibrium is reached in less than a day. Conversely, these small detection packets have high rates of desorbtion which cause the resulting measurements to change significantly dependent upon how long it takes to transmit the packet from the sampling area to the laboratory where the measurements are actually taken. Thus there is good reason to doubt whether these widely-used forms of radon dosimetry are providing accurate measurement of radiation levels for the buildings and homes where they are commonly being used as radiation detectors. Since substantial investment decisions are routinely being made concerning purchase or remedial measures based upon these tests there is a substantial need for better dosimetry systems for measuring radon levels.
The above explanation provides an indication of the continuing need for improved dosimetry systems for heavy particle radiation, particularly neutron, proton and alpha particle radiation. Still further there remains a need for reliable, economical, and accurate radiation dosimetry systems which are capable of measuring heavy particle radiation exposure. There is also a need for such dosimetry systems which can utilize reusable dosimeter detectors which are easily read without costly laboratory procedures.